A stuffer is either a insert you buy that you put in your diaper (before you put it on), or another (typically smaller) diaper that has been disassembled in which is also put in your diaper that is supposed to increase the capacity of said diaper. Typically they do work. Pamperchu has some great tutorials on stuffing diapers with other diapers.

A stuffer usually refers to a smaller diaper that may not fit put inside a larger diaper, like a baby diaper put inside of an adult diaper to increase the thickness feel of the larger diaper as well as the absorbency.